If someone spends £2000 p/m using his AMEX, then in order to breakeven on the premium card vs. the standard, you would have to book trips with AVIOS that utilised a minimum of 2c (£0.02) per AVIOS.
All other benefits being equal, it seems that the standard card will be more beneficial in isolation. This means that if no other benefits are used in a given year and only AVIOS are utilised, then AMEX Standard seems to outperform AMEX Premium given that the avg. Avios cash values are between 0.8c (£0.008) - 1.9c (£0.019)